The present invention relates generally to communication systems and, more particularly, to systems and methods that use asymmetric pairs of mutually orthogonal codes for spreading and despreading transmissions in point-to-point, point-to-multipoint, and multiple access optical communication systems.
A major area of development in optical communications using fiber or directly over the air transmissions involves the implementation of multiple access techniques, where multiple users share the same communication channel. Multiple access techniques that have been implemented in optical networks include the Waveform Division Multiple Access (WDMA) and Time Division Multiple Access (TDMA) techniques.
In WDMA, the network partitions the bandwidth into non-overlapping wavelength (i.e., color) segments (or channels) and allocates the segments to the users in a predetermined manner. In dedicated allocations, each user has the interference-free use of his allocated bandwidth segment at all times. One problem with dedicated wavelength allocation is that if the user""s access to the network is bursty, WDMA may not efficiently utilize the existing bandwidth. Moreover, as new users are added, the network re-allocates the bandwidth resource, which may be neither feasible nor desirable in large-scale networks.
A usual practice is to allocate a wavelength channel to multiple users and use TDMA to control the shared utilization of the wavelength channel bandwidth. In TDMA, the network allocates non-overlapping, time slots to the users so that each has the interference-free use of the entire wavelength channel bandwidth during his allocated time slot. By increasing the number of time slots, new users can be accommodated. Like WDMA with dedicated allocations, the TDMA protocol may lead to inefficient bandwidth utilization in a bursty network. Each user has a very short duty cycle transmission period that requires time slot synchronization for proper operation. Since only one user is allowed to access the network at any time instance, the TDMA protocol is essentially a coordinated, single user system.
A current trend in the industry is to integrate WDMA and TDMA with Code Division Multiple Access (CDMA) spread spectrum communications. CDMA is a type of spread spectrum technique that allows signals from multiple users assigned to a common wavelength channel to overlap in time by assigning different spreading codes to different users. More specifically, CDMA systems xe2x80x9cspreadxe2x80x9d signals across a common communication channel by multiplying each signal with a unique spreading code sequence. A transmitter then transmits the signal on the common channel in overlapping fashion as a composite signal. In CDMA systems using conventional xe2x80x9csymmetricxe2x80x9d spreading-and-despreading sequences, each receiver correlates the composite signal with a respective unique despreading code sequence that is an exact replica of the spreading code sequence used to spread the signal at the transmitter to extract the signal addressed to it.
In conventional optical fiber CDMA communication systems, multiple users transmit simultaneously using binary signaling and non-coherent ON-OFF optical transmission techniques in which the presence of light energy (or the lack thereof) is used to signal the spread sequence xe2x80x9cchipsxe2x80x9d as being either xe2x80x9c1xe2x80x9d or xe2x80x9c0.xe2x80x9d These multiple signals, being available superposed at different receivers with different relative power levels and relative delays, are non-orthogonal to each other.
Receivers use correlation operations to despread and separate the received signals. The use of such correlation operations, however, results in signals being received with large amounts of cross-correlation interference. In order to reduce and even eliminate the amount of associated interference, it is desirable to create a spread spectrum optical communication system in which the orthogonal nature of the codes is maintained at the receivers regardless of the nature of transmissions (coherent or not), relative delays and power levels associated with these signals.
Therefore, there exists a need for a system and method that reduce interference in spread spectrum optical transmissions.
Systems and methods consistent with the present invention address this and other needs by providing a mechanism that reduces the interference of spread spectrum transmissions in an optical multiple-access network by using despreading codes that are different (i.e., asymmetric) and orthogonal to the codes used for spreading.
A method for processing a signal in an optical network includes generating a spreading matrix having a plurality of spreading codes, spreading a signal using one or more spreading codes from the spreading matrix, converting the spread signal to an optical signal, and transmitting the optical signal through the optical network. The method further includes converting the optical signal back to the spread signal, generating a despreading matrix that is an inverse of the spreading matrix and includes a plurality of despreading codes, despreading the spread signal using one or more despreading codes from the despreading matrix, and processing the despread signal.
In another implementation consistent with the present invention, a receiver includes an optical-to-electrical converter, a despreading code generator, and a correlator. The optical-to-electrical converter is configured to receive a transmitted optical signal and convert the optical signal to a digital signal. The despreading code generator is configured to generate at least one orthogonal despreading code, the at least one orthogonal despreading code being different than a code used to spread the transmitted optical signal. The correlator is configured to receive the digital signal and the at least one orthogonal despreading code, and despread the digital signal using the at least one orthogonal despreading code.
In yet another implementation consistent with the present invention, a system includes first and second optical transmitters. The first optical transmitter is configured to generate a spreading code matrix, rows of the spreading code matrix corresponding to different spreading codes, allocate a first group of spreading codes in the spreading code matrix to a first signal to be transmitted, spread the first signal using the first group of spreading codes, convert the spread first signal to an optical form, and transmit the optical first signal. The second optical transmitter is configured to generate the spreading code matrix, allocate a second different group of spreading codes in the spreading code matrix to a second signal to be transmitted, the second different group being allocated such that a third group of unassigned spreading codes is located between the allocated first group and second different group of spreading codes, spread the second signal using the second different group of spreading codes, convert the spread second signal to an optical form, and transmit the optical second signal.